The present invention relates generally to the use of inorganic sorbent materials, and methods of synthesis, for sequestering radioactive waste products, such as strontium (90Sr), and alpha-emitting actinides, such as americium (241Am), plutonium (238Pu, 239Pu, 240Pu), and neptunium (237 Np).
Nuclear materials production in the United States has produced large amounts of highly radioactive wastes. Wastes stored at the Savannah River and Hanford sites exhibit high alkalinity and high ionic strength. Cost effective disposal of the high-level nuclear wastes requires highly efficient radiochemical separations processes. Ion exchange materials have shown promise to separate fission products (e.g., cesium and strontium) and actinide elements (e.g. plutonium and neptunium) from the high-level waste solutions prior to disposal.
Monosodium titanate (MST) is an inorganic, amorphous, poorly crystalline, hydrous metal oxide sorbent material that exhibits high selectivity for strontium and alpha-emitting actinide radioisotopes, even in the presence of strongly alkaline and high sodium-containing salt solutions. The Savannah River Site (SRS) identified MST for removal of strontium and plutonium isotopes from high-level radioactive waste solutions in the early 1980s as part of the In-Tank Precipitation process. In 2000, the Department of Energy (DOE) selected MST for the strontium/actinide separation step. Original development of MST at Sandia National Laboratory (SNL) produced a dried powder. Unpublished studies conducted by L. L. Kilpatrick and D. T. Hobbs during the 1980s indicated that air-drying of the MST at elevated temperature (e.g., >100° C.) adversely impacted strontium removal performance. Principally due to the poorer sorption characteristics of MST dried at elevated temperature, procurement of MST for the In-Tank Precipitation (ITP) process at SRS specified that the vendor prepare and isolate the material without drying, and then deliver the MST to the Westinghouse Savannah River Company (WSRC) as an aqueous slurry containing 15 to 20 wt % MST solids.
Conventional synthesis of sodium titanate, HNaTi2O5, involves reacting a titanium isopropoxide and sodium hydroxide in a molar ratio of 2 to 1. (See, e.g., U.S. Pat. No. 4,156,646, “Removal of Plutonium and Americium from Alkaline Waste Solutions”, to Schulz.) The titanium isopropoxide is added to the sodium hydroxide-methanol solution with stirring. Hydrolysis is done by pouring the titanium isopropoxide+sodium hydroxide mixture into an acetone-water solution containing 8.5% water by volume; one liter of the acetone-water solution being required for each mole of titanium hydrolyzed. The hydrolyzed material is coarse, and can be easily filtered to a 50 micron frit.
Disadvantages of using MST as slurry include the possibility of a chemical spill from a leak in the storage containers or transfer lines, and the cost of additional equipment needed to suspend or maintain suspension of MST solids in the slurry. Advantages of using MST as a dry powder include more precise control of MST quantity, potential longer chemical shelf life, no possibility of liquid spills, lower transportation costs, and smaller storage footprint.
Recent tests by the authors have confirmed the earlier work that air-drying of the MST slurry at elevated temperature (>100° C.) adversely impacts the kinetics of strontium and alpha removal performance (i.e., air-dried MST sorbs more slowly). However, we discovered that air-drying at 100 C did not significantly affect the MST particle morphology or the sorbent's capacity. See “Preparation and Use of Dried Monosodium Titanate”, D. T. Hobbs, M. D. Nyman, Westinghouse Savannah River Company technical report WSRC-TR-2003-00546, which is incorporated herein by reference.
It is well known that the permanent clean up and disposal of radioactive wastes is a very expensive process. Any improvements in the MST sorbent's capacity and/or kinetics could significantly reduce these costs. However, due to the regulatory process, and other concerns, in order to successfully replace the current baseline sorbent (MST slurry) with an improved sorbent, significant increases in performance will need to be shown. Against this background, the present invention was developed.